1. Field of the Invention
The present invention relates to a photothermographic material having a high silver iodide content. More specifically, the invention relates to a photothermographic material that exhibits high sensitivity and low fog, and is excellent in lightfastness of images after processing.
2. Description of the Related Art
In recent years in the medical field, it has been strongly desired to reduce the volume of processing waste fluids from the standpoints of environmental protection and conservation of space. Thus, there has been a need for technologies relating to photothermographic materials which are applicable for medical diagnosis or photographic technologies and can efficiently be exposed by a laser image setter or a laser imager to form clear black images having high resolution and sharpness. These photothermographic materials are advantageous in providing customers with a thermally developing system that does not need liquid-type processing solutions, and which is simple and not harmful to the environment.
There is also a need for the similar technologies in the field of ordinary image forming materials. In particular in the field of medical diagnosis, which requires detail depiction, high quality images excellent in sharpness and graininess are needed and blue black image tone is desired in view of diagnosing readiness. Currently, various types of hard copy systems using pigments and dyes, for example, ink jet printers and electrophotographic systems are widely used as the ordinary imaging system. However, the materials are not yet satisfactory in view of image quality (sharpness, graininess, gradation and tone) for providing diagnostic ability necessary for medical images and recording speed (sensitivity), and they have not yet reached a level at which they can replace existent medical films containing silver sats, that are used in a conventional wet developing system.
On the other hand, photothermographic image forming systems using organic silver salts are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, and in “Thermally Processed Silver Systems” written by D. H. Klosterboer, “Imaging Processes and Materials” written by Neblette, 8th Ed., edited by J. Sturge, V. Walworth & A. Shepp, Chap. 9, p. 279, 1989. Usually, photothermographic materials have a photosensitive layer produced by dispersing a catalytically active amount of a photocatalyst (e.g., silver halide), a reducing agent, a reducible silver salt (e.g., organic silver salt), and optionally a toning agent for adjusting silver color tone in a binder matrix. Photothermographic materials of this type are, after having been imagewise exposed, heated to an elevated temperature (e.g., at 80° C. or higher) to form black silver images through a redox reaction between a reducible silver salt (serving as an oxidizing agent) and a reducing agent. The redox reaction is accelerated by catalytic action of latent images which have been formed on silver halides exposed to light. Therefore, the black silver images are formed in the exposed area (see, for example, U.S. Pat. No. 2,910,377 and Japanese Patent Application Publication (JP-B) No.43-4924).
As a result, the Fuji Medical Dry Imager (FM-DP L) was marketed as a medical diasnostic image-forming system utilizing the photothermographic materials.
There are two methods for manufacturing the photothermographic material utilizing the organic silver salt. The first method involves applying a coating liquid containing the silver salt dissolved in a solvent and the second method involves applying an aqueous coating liquid containing polymer fine particles as a main binder followed by drying. The latter method is advantageous in that it only requires a simple manufacturing equipment since a step for recovering the solvent is unnecessary, making it well-suited for mass-production.
Since such an image forming system utilizing the organic silver salt does not employ a fixing step, it is problematic in that after development, image preservation is adversely affected, especially when the printed out image is exposed to light. In order to improve this printout problem, a method utilizing AgI formed by converting the organic silver salt is described in certain patent publications, e.g., U.S. Pat. No. 6,143,488 and European Patent (EP) No. 0922995. However, these methods of utilizing the conversion of the organic silver salt with iodine were incapable of obtaining sufficient sensitivity, and were thus unable to create practically applicable systems.
Other photosensitive materials utilizing AgI are described in patent publications (e.g., WO97/48014, WO97/48015, U.S. Pat. No. 6,165,705, Japanese Patent Application Laid-Open (JP-A) No. 8-297345, and Japanese Patent No. 2785129), however, these could not achieve sufficient sensitivity or fogging levels, and hence, these materials are unsuitable for actual use as photosensitive materials exposed with lasers.
In order to increase sensitivity of a photographic silver iodide emulsion, a means of sensitization by immersion in halogen receptors such as sodium nitrite, pyrogallol and hydroquinone has been known. Also, immersion in an aqueous silver nitrate solution, or sulfur sensitization at pAg 7.5 have been known. These have been described in publications such as the Journal of Photographic Science, Vol. 8, page 119 (1960), ibid. Vol. 28, page 163 (1980), or Photographic Science and Engineering, Vol. 5, page 216 (1961). However, as shown in the Examples herein, the sensitizing effects exhibited by these halogen receptors were poor in the photothermographic material as disclosed herein.
Doping a photosensitive silver halide with a heavy metal has been carried out on a variety of silver halides. For example, doping a transition metal belonging to Groups VI to X of the Periodic Table into the inside a silver halide grain is described in JP-A No. 2001-42471. It is preferable that a larger amount of the doping material is distributed on a surface of the grain or in the vicinity thereof rather than inside the grain, or is uniformly distributed within the grain so as to exhibit high sensitivity. As such silver halides, silver bromide, silver iodobromide and silver chloroiodobromide are used with a silver iodide content ranging from 0.01 mol % to 10 mol %, and preferably from 0.1 mol % to 5 mol %.
JP-A No. 2000-66325 discloses a method for doping a silver halide grain with an iridium-type dopant and a transition metal dopant other than iridium, localized in mutually different regions, to obtain high sensitivity. In particular, the effect of the method to enhance sensitivity at the time when irradiated with high intensity light is set forth. As the silver halides, similar to the above, silver bromide, silver iodobromide, silver chloroiodobromide and silver iodide are used with a silver iodide content ranging from 0.01 mol % to 10 mol %, and preferably from 0.1 mol % to 5 mol %. It is also described therein that such a doping method is applicable to a multi-color photosensitive material having two or more silver halide emulsion layers, however, there is no mention that this method is applicable to a photothermographic material.
As described above, for use in the silver halide emulsions having a silver iodide content of 10 mol % or less, preferable heavy metals and preferable methods for adding such metals are conventionally known, however, for use in the silver halide emulsions having a high silver iodide content of 40 mol % or more, no metals or methods are known whatsoever. The silver halide emulsion having such a high silver iodide content exhibits characteristics that differ completely from those of the silver halide emulsion having a silver iodide content of 10 mol % or less. When doping of a heavy metal ion is carried out, known technologies pertaining to the silver halide emulsion having a silver halide content of 10 mol % or less cannot be applied, and therefore, development of a new technology has become necessary.
Conventionally, silver halides having a high silver iodide content were not put into practical use, for the reasons detailed above. Accordingly, there has been no interest or motivation in doping of heavy metals, not to mention any interest in applying such doping to photothermographic materials.